1. Field of the Invention
The present invention relates to a video coding apparatus and a video decoding apparatus for encoding a video signal at a high efficiency, transmitting and storing the coded signal, and decoding the coded signal.
2. Description of the Related Art
Generally, a video signal is compression-encoded before being transmitted or stored because the signal has an enormous amount of information. To encode a video signal at a high efficiency, a picture or an image of a frame is divided into a plurality of blocks in units of a predetermined number of pixels. Orthogonal transformation is performed for each block to separate the special frequency of a picture into frequency components. Each frequency component is obtained as a transform coefficient and encoded.
As one video coding system, a video coding system belonging to a category called mid-level coding is proposed in J. Y. A. Wang et. al., "Applying Mid-level Vision Techniques for Video Data Compression and Manipulation", M.I.T. Media Lab. Tech. Report No. 263, Feburary 1994.
In this system, when a picture consists of a background and an object, these background and object are separately encoded.
To separately encode the background and the object, an alpha-map signal (indicating the background by black pixels and the object by white pixels) which is subsidiary video information representing the shape of the object and the position of the object in the frame is necessary. An alpha-map signal of the background can be uniquely obtained from the alpha-map signal of the object.
As methods of efficiently encoding this alpha-map signal, a binary video encoding method (e.g., MMR (Modified Modified READ) encoding) or a line figure encoding method (e.g., chain encoding) is used.
Furthermore, to reduce the amount of codes of an alpha-map, it is possible to use a method (J. Ostermann, "Object-based analysis-synthesis coding based on the source model of moving rigid 3D objects", Signal Process. Image Comm. Vol. 6, No. 2, pp. 143-161, 1994) by which the contour lines of a shape are approximated by polygons and smoothed by spline curves or a method (Japanese Patent Application No. 5-297133) by which an alpha-map is encoded by reducing it and approximated by curves when it is enlarged.
In the above system of separately encoding the background and the object in the frame, the amount of codes is increased by the amount of alpha-maps compared to the conventional coding method of encoding a whole frame at once. This increase in the alpha-map code amount decreases the coding efficiency.